At present, in radio telecommunication systems represented by wireless LAN, mobile telephone systems, and so on, modulation/demodulation schemes, assigned frequencies, and the like are different depending on differences in system and country. Under such a situation, developments are now actively under way for a software radio which can support a variety of systems with one type of radio. In this software radio, it is desired that not only a modulation scheme but also a used radio frequency can be changed by modifications in software. To implement such a function, it is desired that a center frequency and a bandwidth can also be varied in an arbitrary manner in a filter for selecting only a desired frequency band from frequencies of radiowaves which are transmitted/received by an antenna. However, with currently used surface acoustic wave filters, dielectric filters, or laminated ceramic filters, or SAW filters, it is extremely difficult to provide a mechanism for varying the center frequency and bandwidth, as desired in the software radio. Also, a magnetostatic wave filter is known as a device which is a filter that is available in GHz bands and has a variable center frequency, but even with this device, it is difficult to arbitrarily change the bandwidth.
In recent years, as disclosed in JP-2001-94062-A (Reference 1), there has been an increase in developments for narrow-band filters for intermediate frequency bands with the advance of micro-electromechanical system (MEMS) technologies. This type of filter couples a plurality of miniature resonators between the input and the output to pass signals at a desired frequency therethrough and attenuate signals at other frequencies and noise. In a filter which is fabricated using such MEMS technologies, there exist shifts from and variations in a designed value for a center frequency of a resonator associated with the frequency characteristics of the filter; shifts from and variations in a designed value for a coupling coefficient between resonators; and shifts from and variations in a designed value for an external Q-value of a desired frequency signal which couples an input/output unit and a resonator. Reference 1 describes, as a method of adjusting such shifts and variations in the frequency characteristics, that the density of a silicon conductive layer is varied through ion implantation, or that the resonators are changed in thickness. Therefore, the technique described in Reference 1 is incapable of adjusting the frequency characteristics by a movable mechanism which is instructed in software and is mechanically controllable with electric signals.
Frank D. Bannon, III, John R. Clark, and Clark T.-C. Nguyen, “High-Q HF Microelectromechanical Filters,” IEEE Journal of Solid-State Circuits, Vol. 35, No. 4, 512-526 (2000) (Reference 2) discloses a method of adjusting such shifts and variations in the frequency characteristics by a movable mechanism which is integrated by the MEMS technologies and is mechanically controllable with electric signals.
The method of Reference 2 adjusts the center frequency of a resonator by applying a DC voltage between an input/output unit of a desired frequency signal and the resonator for purposes of adjusting shifts from and variations in a designed value for the center frequency, among other frequency characteristics of the filter. In this way, several methods of partially adjusting the frequency characteristics of a filter, limited to the center frequency, have been proposed in the past, as disclosed in Reference 2.
However, in filters using the conventional MEMS technologies, there has not so far been proposed any filter which can adjust shifts from and variations in design values with respect to general frequency characteristics of a filter, i.e., all characteristics such as the center frequency, bandwidth, rejection characteristic, insertion loss, and phase characteristic. For adjusting such general frequency characteristics, it is necessary to adjust, if it concerns a single-stage filter, all three parameters: the center frequency of a resonator, an external Q-value between an input electrode and the resonator, and an external Q-value between an output electrode and the resonator independently of one another. Here, the single-stage filter refers to a filter which has only one resonance mode for use as a filter. In the case of a plural-stage filter which has a plurality of resonance modes for use as a filter, the center frequency of a resonator, a coupling coefficient between the resonance modes, and an external Q-value for coupling an input/output unit of a desired frequency signal with the resonator must be made individually adjustable to their respective design values. For example, while Reference 2 shows that the center frequency of the resonator can be adjusted to a desired value by applying DC voltages between input/output electrodes and resonator to deflect the resonator, this method simultaneously makes the coupling stronger, caused by an electrostatic force, between the input/output unit of a desired frequency signal and the resonator, because the deflected resonator narrows down the spacing between the input/output electrode and resonator. Consequently, when the center frequency is adjusted, the external Q-value is also changed.
Also, even in an MEMS filter which can adjust shifts and variations in part of the frequency characteristics by a movable mechanism fabricated by such MEMS technologies and mechanically controllable with electric signals, there has not so far been contemplated any system which can change the frequency characteristics to support a variety of standards having different frequency bands and center frequencies by varying the frequency characteristics of the filter in an electrically controllable form, as is required by the software radio.